Why do American semi-trucks (18 wheelers etc) exclusively use inline 6-cylinder engines, no matter if it’s a Mack, Detroit Diesel, Cummins, or Volvo-built engine?


I get why they are diesel, but why are they all of the inline 6-cylinder configuration? Why not V6 or straight 8 or freakin huge inline 4-cylinders?

In: 715

It’s most efficient to use inline-6 motors.

It’s a balance between engine strength, ease of maintenance and space for other systems.

They have a more simple design, less parts, smoother and produce lower end torque that is important in a truck. They pull heavy loads, so the low end torque allows them to get up and running without having to wind out the engine, which means lower maintenance since they don’t have to be beaten into action.

Inline-6 cylinder engines have a unique characteristic: they can have perfect primary and secondary dynamic balance without needing a balance shaft.

Essentially what this means is that the forces on the engine caused by the pistons and whatnot moving around mostly cancel each other out, and the engine runs very smoothly without needing extra, very heavy parts to dampen the vibrations.

All other engine types (V8, V6, especially I4) have imperfect balance, and V6 and I4 engines have bad enough balance that they need the heavy balance shafts to reduce vibration. I6 engines (and engines made of I6s like V12s) don’t.

As the mass of the moving parts increases, the forces involved increase, so by the time you get to the size of engine needed to move an 18 wheeler, you’re dealing with some truly massive forces. With anything other than an I6, this would mean monstrous vibration or a stupidly heavy balance shaft that would take a lot of engine power to spin.

Thus, for large engine applications, I6s and V12s are an easy choice.

They did use V-6, V-8 and V-12. Back in the 60’s through the early 90’s. IIRC.

Cat made a Big V-8, Cummins made a big V-6 and Detroit made all of them in a 2-stroke. Detroit even made a V-16.

Recently Walmart began testing on a new opposed piston engine. Its a 2-stroke engine, 3 cylinders and 6 pistons.


Plenty of big V8 diesel trucks still operating here in Australia, but then we have road trains not just a tractor trailer with one trailer, ours have up to three trailers so they need the extra power to pull all that weight.